Biaxially oriented high density polyethylene (OHDPE) films used for packaging, decorative, and label applications often perform multiple functions. It must perform in a lamination to provide printability, transparent or matte appearance, or slip properties; it sometimes must provide a surface suitable for receiving organic or inorganic coatings for gas and moisture barrier properties; it sometimes must provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or laminating.
Heat seal ability can be achieved by liquid coating a heat sealable skin on top of the core layer of the OHDPE film or by extrusion coating a heat sealable skin on top of the core layer of the OHDPE film, but these processes usually require special treatments on the receiving surface of the OHDPE film, and/or a tie layer between the OHDPE core and the heat seal layer, which add cost and complexity to the manufacturing process, and often may sacrifice other properties of the OHDPE film such as gloss and haze. Another way to obtain heat sealability in BOPP films is by co-extrusion of a heat sealable skin on top of a core layer of the OHDPE film, but the heat seal strength obtained in such cases is generally low and limited to a narrow temperature range.
U.S. Pat. No. 5,302,442 describes a heat sealable OHDPE film formulation using a heat seal formulation of about 15-50 wt % of a random terpolymer of ethylene-propylene-butene, about 10-50 wt % of low density polyethylene and about 10-50 wt % of polybutene homopolymer or copolymer, co-extruded to the HDPE core. However, the film has a heat seal strength limited to 500 grams per inch.
U.S. Pat. No. 4,564,558 describes a heat sealable OHDPE film formulation comprising a polyolefin film substrate, a layer of a terpolymer of ethylene, propylene and 1-butene, a primer layer on said terpolymer layer and a heat sealable layer on said primer layer, where the heat seal strength reached values above 800 g/in. However, the film requires a primer coating on the terpolymer layer and a subsequent heat seal coating on top of the primer coating, which add cost and complexity to the process.
There remains a need in the art for new and improved methods for making OHDPE films with new and improved heat sealing properties. The inventor has found a solution that improves the attribute of heat sealability of the OHDPE film by adding an amount from 1-9 wt % of a nucleating agent to the core layer of the film, while also maintaining the optical properties of the OHDPE film, i.e., haze and gloss of the film.